1. Field of the Invention
This invention relates generally to an apparatus, process, and the product produced therefrom for constructing a spunbond, non-woven web from thermoplastic polymers producing filaments of reduced diameter and improved uniformity at an increased production rate, and specifically, to an apparatus and process for heating and extruding thermoplastic materials through a spinneret, forming filaments of finer deniers by strategically positioning the drawing unit below the spinneret at a critical distance to produce a finer filament of a desired diameter and with an improved production rate, and the resultant spunbound product. A water spray for cooling may also be employed.
2. Description of the Prior Art
Devices for producing non-woven thermoplastic fabric webs from extruded polymers through a spinneret that form a vertically oriented curtain with downward advancing filaments and air quenching the filaments in conjunction with a suction-type drawing or attenuating air slot are well known in the art. U.S. Pat. No. 5,292,239 discloses a device that reduces significant turbulence in the air flow to uniformly and consistently apply the drawing force to the filaments, which results in a uniform and predictable draw of the filaments. U.S. Pat. No. 3,802,817 discloses a sucker apparatus positioned in a selected distance below the spinneret using jet streams having velocity in the range of turbulent flow to produce fine non-woven fleeces. U.S. Pat. No. 4,064,605 and European Patent Application No. 0230541 disclose examples of the formation of non-woven fabrics.
Conventionally, thermoplastic polymers such as polypropylene, polyethylene, polyester, nylon, and blends thereof are utilized. In the first step, the polymer is melted and extruded through a spinneret to form the vertically oriented curtain of downwardly advancing filaments. The filaments are then passed through the quench chamber where they are cooled down by chilled air, reaching a temperature at which the crystallization of the filament starts, resulting in the solidification of the filaments. A drawing unit located in a fixed position below the quench chamber acts as a suction having an air slot where compressed air is introduced into the slot, drawing air into the upper open end of the slot, forms a rapidly moving downstream of air in the slot. This air stream creates a drawing force on the filaments, causing them to be attenuated or stretched and exit the bottom of the slot where they are deposited on a moving conveyor belt to form a continuous web of the filaments. The filaments of the web are then joined to each other through conventional techniques.
Providing for conventional construction of the filaments, typically filaments of 1.5 to 6 deniers or higher were produced. Using conventional methods, the hot filaments leaving the spinneret typically were immediately cooled to ambient temperature and solidified and then subjected to the drawing unit. According to a prior proposal, when the length of the filament traveling through the air is shorter than a specified value selected based on the throughput (gram per hole per minute) used, the extruded filaments will contact with solid constituent of the drawing unit in advance of solidification of the filaments, resulting in development of filament breakage or damage. In other words, even though the prior art produces suitable non-woven webs, their production is limited by the ability to cool down and solidify the filaments in a predetermined length at appropriate throughput. The filament spinning speed reached in the prior art is in the range of 3,000 to 3,500 meters per minute.
Although the conventional method and apparatus produce suitable non-woven webs, the final product could be greatly improved and better fabric can be produced consisting of lower denier filaments. A thinner filament produces more surface area and more length per unit weight. A polypropylene spunbonded fabric with filaments of 0.1 to 2.0 deniers would be desirable.
When evaluating the thickness, different types of thermoplastic polymers may require some adjustment in thickness. Slightly varying diameters in other thermoplastic polymers such as polyethylene or polyester may require an adjustment also to consider the production rate.
It is also desirable that a uniformity of denier and tensile properties be consistent so that the resulting fabric web has a uniform quality.
Examples of end uses for the fabric web could be filtration materials, diaper covers and medical and personal hygiene products requiring liquid vapor barriers that are breathable and have air permeability.
With the present invention, a process for producing a superior quality non-woven web at much higher production and lower cost can be achieved. The core of the invention lies in the usage of a technique comprising principally of adjusting the processing variables such as throughput, air pressure, and volume while moving the drawing unit vertically along the spinline towards the spinneret, resulting in the reduction of air drag associated with the length of the filaments traveling with high velocity and causing an increase in drawing force exerted on the filaments of shorter length. The increased drawing force not only produces thinner filaments at higher filament spinning speed, but also creates stronger stress-induced crystallization effect, causing the on-line crystallization of filaments to occur earlier along the spinline at higher temperature and rate. Correspondingly, the filaments are solidified earlier at higher temperature, thereby resulting in less quench capacity needed or higher mass throughput can be used with the same quench capacity. 90 to 95 percent reduction of the air drag associated with the length of filaments between the drawing unit and the spinneret can be achieved by moving the drawing unit from a conventional distance of 3 to 5 meters from the spinneret to 0.2 to 0.5 meters, giving rise to the possibility of producing finer filaments at higher production rate. By changing the position of the drawing unit and utilizing a water mist, the diameter of the filaments can be controlled in such a way that while sticking among filaments in contact can be avoided, the temperature of the filaments remains as high as possible before they enter the drawing unit, reducing the viscosity of the filaments being drawn and consequently facilitating the attenuation of the filaments, resulting in filaments having much smaller diameters. The position of the web forming table corresponding to the drawing unit can also be adjusted in order to form a non-woven web which has desired uniformity with other mechanical properties.
A water mist may be added for interacting in the process to improve the filament uniformity and production. The water mist improves the process, but the basic apparatus and process will work without the water mist solely by the reduced separation of the spinneret and the drawing unit.
In terms of filament spinning speed, 4500 meters per minute for polyethylene terephthalate (PET) and 3500 meters per minute for polypropylene (PP) are achievable in the prior art and in commercial production today. With the Applicant's invention, Applicant believes that 8000 meters per minute for PET and 6400 meters per minute for PP have been achieved. Applicant has been able to produce melt-blown grade filaments (5 to 10 micronometers at spunbound production rates 70 to 150 Kg/H/M width), which is far beyond the capability of conventional production technology.
In accordance with the invention, a correct startup procedure is necessary to establish (ultimately) optimum conditions with the highest filament spinning speeds at corresponding highest throughputs. For example, a process of producing a spunbound fabric of 4.5 denier of PET filament at 4.0 gram per hole per minute (ghm) which amounts to 8000 meters per minute of filament speed cannot be established if the process begins with the drawing unit positioned close to the spinneret of less than 50 cm. The correct startup of the process is to first begin with a much lower throughput, less than 1.0 ghm and using lower air pressure, between 10 to 20 psig so that threading of the filaments through the slot of the drawing unit can be readily accomplished. Once the initial startup under these conditions is established, the air pressure and the throughput are then adjusted coordinately to a desired condition. A stable process can be obtained wherein 4.5 denier PET filaments are produced at 4.0 ghm with the drawing unit positioned 25 cm below the spinneret using 75 psig of air pressure. Applicant has found that Applicant can use distances between the spinneret and the drawing unit between 5 and 150 cm and optimally between 20 to 90 cm separation between the spinneret and the drawing unit. These small distances, however, are only achieved after the startup procedure mentioned above.
There are two distinct changes occurring for the on-line diameter profile as the filament spinning speed increases. First, the rate of reduction in diameter of the melt thread in the upper region of the spinline increases. In other words, the melt thread is thinning much faster at higher spinning speed, creating more surface area to be cooled. Secondly, the position where the filament starts to solidify due to so-called the stress-induced crystallization moves up towards the spinneret. The higher the filament speed, the less the cooling is needed (shorter quench chamber), and the drawing unit can be lifted up along the spinline without causing interruption of the process because the filaments are well solidified before entering the slot of the unit where contacts among filaments are made. When the distance between the spinneret and the drawing unit is decreased, the drag force Fd, which is associated with the length of filaments (dZ) traveling at high speed between spinneret and drawing unit will proportionally be reduced, resulting in increasing inertial force Finert, which leads to even higher filament speed, further thinner filaments and higher solidifying temperature. This in turn allows the drawing unit to be lifted further up. Our results show that depending on the material to be processed and the throughput (gram per hole per minute, referred to as ghm from now on) to be used, the drawing unit can be lifted up as close as 5 to 40 cm to the spinneret at throughput of up to 4 ghm, comparing with 2 to 4 meters being used in commercial production today, that is over 90 to 95 percent of reduction in air drag force which has significant impact on the output of the process in terms of fineness of filaments that can be produced at achievable production rate. The closer the drawing unit from the spinneret, the higher the temperature at which filaments are being drawn and the lower the elongational viscosity will be, which is inversely proportional to the elongation rate. That is, with lower elongational viscosity, higher elongation rate (higher filament speed) can be achieved under the same drawing force.